The present disclosure generally relates to a method of measurement, and specifically to a method of measuring the strain in a CMOS device with stress-generating elements using X-ray diffraction (XRD), and structures for effecting the same.
When stress is applied to the channel of a semiconductor transistor, the applied stress and the resulting strain on the semiconductor structure within the channel affects the band gap structure (i.e., breaks the degeneracy of the band structure) and changes the effective mass of carriers. The effect of the stress depends on the crystallographic orientation of the plane of the channel, the direction of the channel within the crystallographic orientation, and the direction of the applied stress. Under stress applied to the channel of the MOSFET, the mobility of carriers, and as a consequence, the transconductance and the on-current of the transistor are altered from their original values for an unstressed semiconductor.
The effect of uniaxial stress, i.e., a stress applied along one crystallographic orientation, on the performance of semiconductor devices has been extensively studied in the semiconductor industry. For a p-type MOSFET, i.e., a PMOSFET (or a “PFET” in short) that utilizes a silicon channel or silicon-germanium alloy channel, the mobility of minority carriers in the channel (which are holes in this case) increases under longitudinal compressive stress along the direction of the channel, i.e., the direction of the movement of holes or the direction connecting the drain to the source. Conversely, for an n-type MOSFET, i.e., an NMOSFET (or an “NFET” in short) that utilizes a silicon channel or a silicon-germanium alloy channel, the mobility of minority carriers in the channel (which are electrons in this case) increases under longitudinal tensile stress along the direction of the channel, i.e., the direction of the movement of electrons or the direction connecting the drain to the source.
When a material is compressed in one direction, the material tends to expand in the other two directions perpendicular to the direction of compression. Conversely, if the material is stretched rather than compressed, the material tends to contract in the directions transverse to the direction of stretching. This phenomenon is called the Poisson effect, and is characterized by Poisson's ratio ν. The Poisson's ratio is the ratio of the fraction (or percent) of expansion in one direction divided by the fraction (or percent) of compression in another direction, for small values of these changes. Thus, strain along one direction is invariably coupled with strain along other directions in a single crystalline material, and an accurate measurement of strain in one direction not only provides data on the strain and stress in the crystallographic direction along which the measurement is taken, but also significantly contributes to accurate estimation of the stress and strain along all other crystallographic orientations.